1. Field of the Invention
The invention generally relates to planar light devices for liquid crystal display (LCD) devices.
2. Description of Prior Art
In general, an LCD device has the advantages of thinness, light weight, and low power consumption. For this reason, LCD devices are widely utilized in various types of electronic equipment, from pocket calculators to large-scale office automation equipment.
Conventionally, planar light devices are designed for use in an LCD device for backlighting purposes. The core element of the planar light device is a light guide plate (LGP). The LGP is generally a rectangular, transparent substrate. The LGP generally includes a light incident surface, a light-emitting surface and a bottom surface. Light emitted by a light source is guided to the light-emitting surface, then emits uniformly from the light-emitting surface. A plurality of dot patterns is disposed on the bottom surface, for making light emitted from the LGP more uniform. Such dot patterns for an LGP are disclosed in U.S. Pat. No. 5,363,294.
However, one problem with this kind of LGP is that a mirror image of the dot patterns can be see from the light-emitting surface, resulting in decreased brightness and uniformity of the LGP. In order to solve this problem, two diffusing sheets are generally added onto the light-emitting surface of the LGP, for attenuating the mirror image and improving a uniformity of light emitted from the LGP. However, this solution increases the cost of the planar light device.
U.S. Pat. No. 5,703,667 issued to Ochiai discloses an LGP utilizing the phenomenon of diffraction based on wave optics of light. Referring to FIG. 5, the LGP 2 is disposed within a planar surface illuminator 1. The LGP 2 has a diffraction grating 3 printed or worked on a bottom surface 2b thereof, and a light incident surface (not labeled) adjacent to a fluorescent tube 4. A ratio of a width of a grating part 3 relative to a width of an adjacent non-grating part 3′ progressively increases with increasing distance away from the light incident surface. This enhances a uniformity of light intensity at a top surface 2a of the LGP 2.
However, the LGP 2 encounters certain problems as follows:
First, the uniformity of brightness of the LGP 2 is generally not satisfactory. This is because the lines of the diffraction grating 3 on the bottom surface 2b are parallel to each other, and parallel to the fluorescent tube 4. Thus, the diffraction grating 3 substantially diffracts the light rays emitted by the fluorescent tube 4, whereupon the diffracted light rays emit from the top surface 2a in a first direction which is orthogonal to the alignment of the diffraction grating 3. Other light rays are incident on the diffraction grating 3 in a second direction being parallel to the alignment thereof, and are diffracted only a little if at all, whereupon the diffracted light rays emit from the top surface 2a in substantially the second direction. Thus, only a little of the total light emits from the top surface 2a in the second direction. This results in a view angle problem; i.e., the light intensity of the LGP 2 in the second direction is far less than the light intensity in the first direction.
Second, the diffraction generated on the bottom surface 2b is inefficient. This is because there is a plurality of non-grating parts 3′ on the bottom surface 2b of the LGP 2 interspersed among the diffraction gratings 3. The non-grating parts 3′ cannot diffract light rays impinging thereon, thus decreasing an overall brightness of the LGP 2.